fetchgit: Require a content hash

Without this, the result will not be a fixed-output derivation and
won't work in general.

pkgs/build-support/fetchgit/default.nix

@@ -23,6 +23,8 @@
    server admins start using the new version?
 */
 
+assert md5 != "" || sha256 != "";
+
 stdenv.mkDerivation {
   name = "git-export";
   builder = ./builder.sh;